1. Field of the Invention
This invention relates to a direction finding method and a direction finding system for locating the direction of a wave transmission source from a receiver. The invention also relates a transmitting apparatus and a receiving end apparatus for the direction finding method and system.
2. Description of the Related Art
Heretofore, when conducting communication between a transmission source on a moving body, such as an automobile, a ship or an airplane, and a fixed or moving receiver, and vice versa, it has occasionally been necessary to orient a directional antenna at the receiver in the direction of the transmission source to improve the receiving ability.
In navigation aiding systems in which an electromagnetic wave transmitted from a transmission source disposed at a fixed point on the land is received on a moving body such as an airplane or a ship to locate a cruising position and direction, it is a necessity to measure the direction of the transmission source with accuracy.
In that case, since the transmission source's direction from the receiver varies due to the movement of the transmission source or the receiver, it is necessary to measure the direction of the transmission source at the receiver just before or during communication.
FIG. 5 of the accompanying drawings shows one example of the conventional direction finding system. As shown in FIG. 5, a sending station 2 comprises a pilot signal transmitter 21 equipped with a pilot signal transmitter antenna 23, and an electromagnetic wave transmitter 32, as a communication link, equipped with a transmitter antenna 24. The pilot signal transmitter 21 includes an oscillator 29 and an amplifier 30. The wave transmitter 32 includes a signal input terminal 25, a modulator 26, an oscillator 27 and an amplifier 28.
On the other end, a receiving station 1 comprises direction-finding receiver antennas 12a, 12b, a directional finding receiver 11, an averaging circuit 18, a directional antenna driver 14, a directional antenna 15, a receiver 16 and a signal output terminal 17.
The two direction-finding receiver antennas 12a, 12b of FIG. 5 are arranged in a plane in which the direction of the sending station 2 varies with respect to the receiving station 1; this arrangement is to find out a single dimensional direction.
The receiving station 1 is designed in association with any monopulse method, i.e., an amplitude comparison monopulse method, a monopulse method using a dual-mode spiral antenna, an amplitude/phase comparison monopulse method or a phase comparison monopulse method.
With this system, the sending station 2 transmits a pilot signal 3 to the receiving station 1. The receiving station 1 receives a pilot signal 3 on the direction-finding receiver 11 via the direction-finding receiver antennas 12a, 12b.
Using an amplitude comparison monopulse method, a monopulse method using a dual-mode spiral antenna, an amplitude/phase comparison monopulse method or a phase comparison monopulse method, the direction-finding receiver 11 generates a direction information signal of the sending station 2 from the amplitude ratio and the phase difference of the pilot signal 3 between the two antennas. This signal is supplied to the directional antenna driver 14 via the averaging circuit 18.
The directional antenna driver 14 orients the directional antenna 15 toward the direction of the sending station 2 according to the direction information signal. Thus the directional antenna 15 receives a direct wave 47 from the transmission antennas 24 and transfers its signal from the receiver 16 to the signal output terminal 17.
Generally, the direction-finding receiving system should use an antenna which is large in beam width and hence is low in gain, in order to cover a relatively wide area. However, to improve the S/N ratio, it is necessary to reduce the pass band of the system. Further, it is desirable to simplify the pilot signal transmitter 21. For this purpose, it has hitherto been a common practice to use a non-modulated continuous wave for the pilot signal 3.
In this conventional direction finding system, an error in the direction information from the pilot signal 3 would easily come out due to the multipath interference which occurs in an electromagnetic wave propagation path between sending station 2 and the receiving station 1.
Namely, because a direct wave 37 from the pilot signal 3 and a reflected wave 31 from the ground surface 5 concurrently strike the direction-finding receiver antennas 12a, 12b, a multipath interference would occur.
For minimizing the multipath interference, it is known to time-average the received signals for a relatively long time, smooth the changes of interference with time, which changes are associated with the movement of the moving body, and utilize this average value as direction information.
In this prior method, however, since fluctuations in direction information are ruled over by the pattern of movement of the moving body, the multipath interference would be fixed if the moving body is stopped. Consequently, an error not varying with time would occur so that the result of averaging cannot be obtained.
In addition, to improve the S/N ratio with a reduced pass band of the direction-finding receiver, it is needed to provide a frequency-stable oscillator, thus increasing the cost of production. This is particularly a serious problem when the direction finding is adopted with microwave bands.